video: fbdev: sm712fb: Fix crash in smtcfb_write()

When the sm712fb driver writes three bytes to the framebuffer, the
driver will crash:

    BUG: unable to handle page fault for address: ffffc90001ffffff
    RIP: 0010:smtcfb_write+0x454/0x5b0
    Call Trace:
     vfs_write+0x291/0xd60
     ? do_sys_openat2+0x27d/0x350
     ? __fget_light+0x54/0x340
     ksys_write+0xce/0x190
     do_syscall_64+0x43/0x90
     entry_SYSCALL_64_after_hwframe+0x44/0xae

Fix it by removing the open-coded endianness fixup-code.

Signed-off-by: Zheyu Ma <zheyuma97@gmail.com>
Signed-off-by: Helge Deller <deller@gmx.de>